The gross process of cross coupling reactions can be described as
wherein    R and R′ represent organic groups to be coupled with a C—C bond,    M is the metal component of the catalyst complex,    L represents the ligands present in the catalyst complex,    n is the number of ligands present,    X is a leaving atom or group (e.g. Cl, Br, I, triflate, mesylate, tosylate), and    M′ is a metal or metal-containing group corresponding to the type of the cross coupling reaction concerned (e.g. this metal component is boron for Suzuki-Miyaura coupling, copper for Sonogashira coupling, magnesium for Kharash coupling, silicon for Hiyama coupling, tin for Stille coupling, zinc for Negishi coupling, etc.).
The general mechanism of cross coupling reactions is shown below.

However, from the aspects of practical utilization these methods have some disadvantages, which are particularly pronounced in the field of pharmaceutical industry. One of them is that rather high amounts of catalyst (1-5 mol % related to the substrate) are required, furthermore metal impurities originating from the catalyst can be removed from the end product generally only by tedious and expensive operations. This latter is particularly valid for palladium catalysts, which, in addition, are highly liable to decomposition. As an example, when palladium(0)-tetrakis(tri-phenyl-phosphine) of formula (II),
which is still an industrially frequently used catalyst, is stored in air at room temperature, a considerable amount of palladium black separates within a short time, thus it is advisable to store it in a refrigerator under argon atmosphere. Although cross coupling reactions utilizing the catalyst of formula (II) are performed under inert atmosphere, separation of palladium black is still common, which causes not only a considerable loss in catalyst, but tedious time-consuming and expensive purification steps should also be introduced.